Recent research has shown that increased levels of exhaust gas recirculation (EGR) in spark ignition engines can enable operation at higher compression ratios and loads than were previously possible, due primarily to a reduction in knock tendency. Increasing the amount of dilution by increasing the air/fuel ratio has also been shown to have similar effects.
Implementation of these features gives rise to the problem of ignition and flame propagation at these increased dilution levels. Several companies now sell enhanced ignition circuits and new types of igniters to improve ignitability and promote faster burn rates in the engine. Two examples are “plasma jet” and “railplug” igniters.